Growth and Survival of Larval Fishes: Roles of Competition and Zooplankton Abundance

Abstract

-Interactions among larval gizzard shad Dorosoma cepedianum, bluegills Lepomis macrochirus, and their zooplankton prey were examined in a controlled mesocosm experiment and by field sampling. In the mesocosm experiment, gizzard shad growth and survival were negatively correlated with gizzard shad density and positively correlated with macrozooplankton prey. Bluegill growth was positively correlated with prey availability, but survival was uniformly high despite differences in zooplankton abundance and fish density. Macrozooplankton and copepod biomasses were negatively correlated with fish density. In Lake Shelbyville, co-occurrence of larval gizzard shad and bluegills in the limnetic zone was limited to a 3-week period, and the period of greatest larval gizzard shad abundance preceded the appearance of bluegills. Zooplankton abundance declined greatly after the peak in larval gizzard shad abundance and remained low when bluegills were present. Growth rates of gizzard shad were highest early and declined throughout the summer, whereas bluegill growth was highest during mid to late summer. Growth rates of gizzard shad and bluegills in the field were not correlated with fish density. However, as in the mesocosm experiment, zooplankton biomass was negatively correlated with fish density, bluegill growth was correlated with the abundance of zooplankton prey, and bluegill survival was uniform through time and not related to fish density or zooplankton abundance. Diet overlap was substantial; gizzard shad and bluegills fed selectively on smaller prey items in June, switching to larger cladocerans and copepods by July. Our results suggest that growth and survival of planktivorous larval gizzard shad and growth of larval bluegills are affected by availability of zooplankton prey, which may become limiting when larval fish densities are high.